The invention relates to a hydraulic torque motor comprising a housing with a central boring, an annular piston, which can be moved axially in the housing's boring, but which is prevented from rotating about its longitudinal axis in this boring and a substantially cylindrical rotor which extends axially through the piston, and which can rotate about its longitudinal axis in the housing, but cannot be moved axially in relation thereto, there being provided in the rotor's outer surface at least one spiral groove and the piston has at least one engagement element which extends radially into the groove, the housing's boring together with one end section of the piston partially defines a first cylinder space and together with the piston's second end section partially defines a second cylinder space, the cylinder spaces are arranged for alternate connection with a source and a reservoir for a pressure fluid for movement of the piston axially between two end positions in the housing and thereby rotation of the rotor between two associated angle positions via the engagement element, at each end of the piston there is a radially outwardly projecting end flange which is arranged for sealing and sliding abutment against the housing's boring, and the housing has a centre flange which at the central area of the housing's boring, considered in the axial direction, projects radially inwards, and which works in conjunction with a section of the piston between the end flanges.